For the uninitiated the Irem M92 is an arcade platform introduced in 1991 by Irem.Technically speaking the system is  made from a top and bottom board, the top (main) board is common across the games, but the bottom (game) board can vary game to game.Each game has an encrypted sound cpu.Here are hardware specs:

 

• CPU : V33 @ 9 MHz, V30 @ 7.159090 MHz
• Sound Chip : YM2151 @ 3.579545 MHz, GA20 @ 3.579545 MHz
• Other Chip : GA21, GA22

List of games on this platform:

• Blade Master / Cross Blades! (1991)
• Dream Soccer 94 (1994)
• GunForce (1991)
• Gunforce 2 / Geostorm (1994)
• Hook (1992)
• In the Hunt / Kaitei Daisensou (1993)
• Lethal Thunder / Thunder Blaster (1991)
• Major Title 2 / The Irem Skins Game (1992)
• Mystic Riders / Gun Hohki (1992)
• Ninja Baseball Batman / Yakyuu Kakutou League-Man (1993)
• Perfect Soldiers / Superior Soldiers (1993)
• R-Type Leo (1992)
• Undercover Cops (1992)

With these premises I begun my troubleshooting on an Undercover Cops and GunForce PCBs.

I bought the first as faulty:

Seller warned me about severe corrosion in some area of PCB, this was confimed once received the board, it seems like the board has been stored in a very humid place :

Anyway, once powered it up I got this:

Game actually played blind.I noticed that if I pressed the PAL marked “M92-A3-M” @IC11  graphics were restored in a bad way though:

This was not a case since this PAL was located in the corroded area so I decided to remove the socket.Underneath I found this scenario:

After cleaning from corrosion, I checked all the connections and found a missing contact bewteen pad of pin 8 (an input of the PAL) and its trace which I promptly patched with some kynar wire:

With this fix graphics were almost good but there were jailbars all over the screen:

Again, pressing a specific area of the PCB restored graphics temporarily so I started to look around until I was able to locate the cause of that issue:

Several pins of a custom ASIC on the video board were lifted.Since the pads were corroded too,a reflow was not enough to ensure a good soldering so I decided to reinforce it using some pieces of AWG3o wire :

This fixed this first M92 board completely:

So I moved on the GunForce PCB:

First thing I noticed after my visual inspection was the lack on many electrolytic capacitors in the audio circuit:

Board booted up but video as disturbed and a rustling sound was present on backgorund :

Usually this issues are caused by increased ESR of elecrolytic capacitors which inject disturbs on video circuit as well.The previous repairer thought about this and therefore removed all the electrolytic capacitors but this didn’t cure the problem.But He forgot to check the main amplifier:

Probing in AC the +12V line with my scope confirmed that ripple was present:

If I excluded the +12V from my supergun the video became normal and no more rustling sound.So, since on PCB there was no other component connected to this supply line, the amplifier was most likely the culprit.I removed it :

After fitting a good amplifier and all the missing capacitors, the video and sound were perfect but other two issues were present : the controls didn’t work and sprites were glitched.As for first issue, it was due the fact someone replaced a couple of custom resistor arrays with normal 4.7Kohm ones:

Like the ‘8M472J221J’ part name said, the original array is a mix of 4.7KOhm and 220 Ohm resistors.Not having other spare parts, I opted for a custom solution keeping the 4.7Kohm array and mounting some SMT 220 Ohm resistors on solder side:

This worked perfectly, controls were responding  so I moved on to troubleshoot the sprites issue:

Sprite devices were four 2Mbit MASK ROMs (compatible with 27C020 EPROM) on video board:

I reprogrammed a 27C020 for each ROM file in order to do piggybacking.When I did it on the one @IC41 glitches vanished.I removed the device:

My programmed warned me about a bad contact on pin 31 while I was reading it:

I socketed and replaced it :

100% fixed!End of this double Irem repair.

After Lifeforce, another awesome Konami shoot ’em up on the bench, it’s the turn of Gradius II – GOFER no Yabou :

Both CPU and video board were in great shape but this is what I got once powered it up:

Board sat down on the above message and very rarely showed an ‘ADDRESS ERROR’ or ‘ILLEGAL INSTRUCTION’ message:

Thanks to my friend Josef who sent me a good Gradius II boardset I could narrow the fault in the CPU board but, before knowing this, during my troubleshooting I found with my logic comparator a couple of faulty Fujitsu TTLs, a 74LS32 @7W on video board and a 74LS157 @8G on CPU board  :

So I could concentrate exclusively on CPU board since I know the fault was there for sure.Probing the board with my oscilloscope I found some abnormal activity on a couple of 6264 RAM @10E and 10G :

Launching the game on MAME I could figure out that these two RAMs are used by both the main and slave 68000 CPU :

So a failure in them would explain the missing boot.Not being able to determine which chip was actually faulty, I desoldered both and added sockets:

The one @10G didn’t pass the out-of-circuit test failing in address 1073:

Finally the board could properly boot and enter in game but with missing graphics and crashing after few seconds all the time :

Probing around again with my logic comparator I found a 74LS74 @6F with bad outputs, once removed it failed :

This fixed the board completely, no other issue were present.Evil Konami defeated again.The battle goes on.

 

Another Out Zone PCB (this time on usual ‘TP-018’ hardware) on the bench:

Board was in good shape but I noticed it was heavily reworked on solderside, a lot of ICs were replaced and PCB was not cleaned from solder flux residuals:

Besides, some ground pins of JAMMA edge connector were partially missing or burnt (sign that something gone shorted) and a 100 nF mylar capacitor was missing.I promply restored all of them:

Board booted into game but some sprites were wrong (more than a palette problem it seemed to me an issue related to data bus as if sprites were missing some layers) :

When I went to read the four 1Mbit ROMs containing relevant data I noticed devices were put in wrong sockets:

Silkscreening under the sockets clearly tells where these MASK ROMs must be placed:

But also with ROMs in correct position some sprites were still wrong.I started to test the part of circuit involved in sprites generation and all was fine until I came across two data lines shorted on a 6264 RAM @10A :

The two data lines are connected to the sprites generator custom ‘FCU-2’ and to inputs of a couple of 74LS374 so the short was present also on them.Lifting a pin of the custom didn’t clear the short.

Obviously due the not high resolution of a simple multimeter I could read always a dead short on each of this component.So I decided to use my Polar Toneohm 850A short locator (essentially an audible milliohmmeter) in order to locate the minimum resistance point.

1,110 Ohm measured on the two shorted data lines (pin 12 and 13) of the 6264 RAM @10A:

Around 700 mOhm measured on the two pins of the custom ASIC ‘FCU-2’:

19.3 mOhm on the 74LS374 @19E

17.2 mOhms on the 74SL374 @19C and beep was squeeling at higher frequency :

 

So I was approaching to the short, it was in the area of the two 74LS374.On part side I didn’t notice anything abnormal so I went to inspect the solder side and after some time I find the culprit:

Two pads connected to  inputs of the two 74LS374 were bridged, a ‘kind’ gift of previous repairer.I warmed up my soldering iron and removed it.Powered up the board again :

Board 100% fixed.Mission accomplished.